In industrial robots, comprising two robot parts pivotally arranged relative each other, powerful power consuming motors, which execute the pivoting of the robot, are required. Powerful power consuming motors are large, heavy and expensive, which commands a need of alternative solutions. One alternative is to supplement the robot with a device, which in the pivoting of the robot participates in the pivoting by absorbing the torque during the pivoting from a rest position/initial position, i.e. when the robot starts a work cycle. The concept pivoting from a rest position/initial position refers to a pivoting in a direction where the attraction of gravity contributes to the pivoting. The arrangement is of such a nature that it during the pivoting from the rest position generates a torque, which acts to restore the robot to its rest position/initial position and thereby helps/relieves the driving motor concerned during the lifting/pivoting back. The concept pivoting back to the rest position/initial position refers to a pivoting which counteracts and thereby compensates for the attraction of gravity, which pivoting in the following is designated balancing. The arrangement according to the foregoing is thus considered a balancing arrangement.
By arranging industrial robots with balancing arrangements, which help and relieve the driving motors, the robot manufacturer is not forced to install unnecessarily large and powerful motors in the robot. The opposite also applies, a powerful driving motor in combination with a powerful balancing arrangement increases the lifting capacity in the wrist of a large industrial robot. However, this leads to an increase in dead weight of both the motor and the balancing arrangement, which in turn calls for even bigger demands on the driving motor in question.
A balancing arrangement thus helps the motor concerned to counterbalance the applied handling weight as well as the present dead weight when pivoting occurs during operation of the robot. Balancing arrangements generally consist of weights, gas-hydraulic devices or spring devices in the form of helical springs, torsion springs and/or gas-based balancing cylinders. Apart from the counterweights the above mentioned devices are expensive, heavy and sensitive constructions. Gashydraulic devices are space demanding and are furthermore marred by density problems.
In helical spring-based balancing cylinders there is always a risk for obliquity of the piston rod in relation to the cylinder, the so-called drawer effect, which when it arises leads to wear of the cylinder device and drastically shortened life-time. The alternative with counterweights also results in disadvantages, because a robot with a counterweight can not be as compact and space-saving. The counterweight also restrains the freedom of movement of the robot. When the robot performs overly limited motion cycles, i.e. the robot is moving too little, problems with poor lubrication in the integral bearings arise.
The Japanese patent JP 10015874 discloses a robot arranged with a gravitation compensating spring device. The device comprises a spring housing, which includes a helical spring, a spring seat and a pull rod attached to the spring seat. Three guide pins are each arranged through a separate hole in the spring seat, which glides along the guide pins when the pull rod is pulled out and the helical spring by that is compressed. The aim is to prevent damage on the pull rod.
Industrial robots usually consist of a robot foot, a stand and a robot arm. The stand is rotatably arranged on the robot foot. The robot arm is pivotably arranged in a joint on the stand. The robot arm is composed of arm parts pivotably arranged in relation to each other. The robot arm comprises e.g. a first and a second arm part and also a wrist arranged with a tool attachment. The arm in its initial position/rest position is oriented with the first arm part almost vertical. When the robot is moving/in operation the arm pivots in relation to the stand at the same time as the arm parts pivots in relation to each other.
The total load on the robot consists of the applied handling weight in the wrist on one hand and the present dead weight of the robot on the other. In pivoting, the motor in question pivots the robot arm, and the the gravity acting on the arm loads/influences the balancing arrangement, whereby the balancing arrangement generates a torque.
The balancing arrangement then facilitates for the motor to pivot the arm back to its initial position/rest position. The pivot motor in question must thus, in pivoting the robot back, be able to handle a remaining torque, which is the sum of the moment from the total load as well as the oppositely directed torque generated in the balancing arrangement. The torque generated by the balancing arrangement and the power of the concerned pivot motor are thus in a state of dependence.
The development of industrial robots is moving towards ever larger robots. 10 years ago large robots managed to lift up to 100 kg with the wrist. The further development has made lifts of 200 kg possible and now there is a need to increase the lifting capacity in the wrist to extremely high loads of approximately 250 kg. With as high loads as that in the wrist, it is immensely important that a balancing arrangement works in the right manner.
In balancing arrangements comprising helical springs the helical spring is compressed or extended. When a helical spring is compressed there is always the risk that it deflects sideways i.e. bends/collapses. It must thus be prevented that the helical spring bends.
With loads on the robot of up to 250 kg in the wrist a balancing arrangement is forced to work with very large torque forces and is easily damaged. The damages usually originate through imbalance in the load of the balancing arrangement. In the case with a helical spring, spring housing, spring seat and a pull rod an imbalance in the load of the pull rod, so-called drawer effect, leads to obliquity of the pull rod in the spring housing, wear appears and the expected life-time of the balancing arrangement is reduced to an unacceptably low level. This leads to unwanted and expensive production interruptions. Furthermore there will also be extra unwanted cost for spare equipment.
In the arrangement according to the above mentioned Japanese patent the spring seat cannot turn axially in the spring housing. Load leads to large bending moments in the pull rod part in the spring housing, which results in very high strain in the construction, high surface pressures are generated and all this taken together results in the deflection of the pull rod.
Consequently, when producing industrial robots of the kind described above the need of a balancing arrangement arises, which can manage loads up to 250 kg and at the same time has as long expected life-time as the industrial robot. Thereby unwanted production interruptions and the need of spare parts is eliminated.
Those needs cannot be fulfilled by the balancing arrangement according to the above mentioned Japanese patent.